[PATCH] c++: Suppress error when cv-qualified reference is introduced by typedef [PR101783]

[nick huang <nickhuang99@hotmail.com>]

Reference with cv-qualifiers should be ignored instead of causing an error 
because standard accepts cv-qualified references introduced by typedef which 
is ignored.
Therefore, the fix prevents GCC from reporting error by not setting variable
"bad_quals" in case the reference is introduced by typedef. Still the 
cv-qualifier is silently ignored. 
Here I quote spec (https://timsong-cpp.github.io/cppwp/dcl.ref#1):
"Cv-qualified references are ill-formed except when the cv-qualifiers
are introduced through the use of a typedef-name ([dcl.typedef],
[temp.param]) or decltype-specifier ([dcl.type.decltype]),
in which case the cv-qualifiers are ignored."

PR c++/101783

gcc/cp/ChangeLog:

2021-08-27  qingzhe huang  <nickhuang99@hotmail.com>

	* tree.c (cp_build_qualified_type_real):

gcc/testsuite/ChangeLog:

2021-08-27  qingzhe huang  <nickhuang99@hotmail.com>

	* g++.dg/parse/pr101783.C: New test.

diff --git a/gcc/cp/tree.c b/gcc/cp/tree.c
index 8840932dba2..7aa4318a574 100644
--- a/gcc/cp/tree.c
+++ b/gcc/cp/tree.c
@@ -1356,12 +1356,22 @@ cp_build_qualified_type_real (tree type,
   /* A reference or method type shall not be cv-qualified.
      [dcl.ref], [dcl.fct].  This used to be an error, but as of DR 295
      (in CD1) we always ignore extra cv-quals on functions.  */
+
+  /* PR 101783
+     Cv-qualified references are ill-formed except when the cv-qualifiers
+     are introduced through the use of a typedef-name ([dcl.typedef],
+     [temp.param]) or decltype-specifier ([dcl.type.decltype]),
+     in which case the cv-qualifiers are ignored.
+   */
   if (type_quals & (TYPE_QUAL_CONST | TYPE_QUAL_VOLATILE)
       && (TYPE_REF_P (type)
 	  || FUNC_OR_METHOD_TYPE_P (type)))
     {
-      if (TYPE_REF_P (type))
+      // do NOT set bad_quals when non-method reference is introduced by typedef.
+      if (TYPE_REF_P (type)
+	  && (!typedef_variant_p (type) || FUNC_OR_METHOD_TYPE_P (type)))
 	bad_quals |= type_quals & (TYPE_QUAL_CONST | TYPE_QUAL_VOLATILE);
+      // non-method reference introduced by typedef is also dropped silently
       type_quals &= ~(TYPE_QUAL_CONST | TYPE_QUAL_VOLATILE);
     }
 
diff --git a/gcc/testsuite/g++.dg/parse/pr101783.C b/gcc/testsuite/g++.dg/parse/pr101783.C
new file mode 100644
index 00000000000..4e0a435dd0b
--- /dev/null
+++ b/gcc/testsuite/g++.dg/parse/pr101783.C
@@ -0,0 +1,5 @@
+template<class T> struct A{
+        typedef T& Type;
+};
+template<class T> void f(const typename A<T>::Type){}
+template <> void f<int>(const typename A<int>::Type){}